Recently, due to growing interest in an environmental problem, it has been demanded further more reduction of an amount of NOx emission from an automobile. Then, various kinds of NOx after treatment devices such as a lean NOx trap catalyst for gasoline engine or a selective catalytic reduction (SCR) for diesel engine have been actively researched and developed. In order to evaluate NOx after treatment devices, a measurement of various nitrogen compounds such as NO, NO2, N2O, NH3 gets attention. Especially, for evaluation of the selective catalytic reduction (SCR), the measurement of NH3 becomes important to evaluate NH3 slip. In addition, since there is a regulation on NH3 in the EURO VI scheduled to be enforced in 2014 in EU, a measurement that can be conducted at a low concentration of NH3 with high response speed is required.
As shown in the patent document 1, a conventional gas analyzer used for measuring a concentration of NH3 introduces an exhaust gas emitted from an exhaust pipe of an automobile into a measurement cell by means of a sampling pipe and measures the concentration of NH3 by the use of a light absorbance of NH3.
However, with the gas analyzer of this sampling method, there is a problem that NH3 contained in the exhaust gas is highly adherent and attaches to an inner wall of the sampling pipe or an inner wall of a flow rate control device arranged on a pipe. As a result, there is a problem that it is difficult to measure the concentration of NH3 with high accuracy. Especially, the concentration of the NH3 contained in the exhaust gas is low so that it takes time to introduce the exhaust gas into the measurement cell due to the inner wall of the pipe. As a result of this, the response speed is lowered so that there is a problem that it is difficult to conduct the measurement at a high speed.
In addition, with the gas analyzer of this sampling method, a suction pump is arranged between the sampling pipe and the measurement cell to sample the exhaust gas and to introduce the sampled sample gas into the measurement cell.
However, in case that the exhaust gas is sampled by the use of the suction pump, a pressure of inside of the sampling pipe fluctuates between a negative pressure and a positive pressure by the pressure of the exhaust gas emitted from the exhaust pipe (a tail pipe). In case that the pressure becomes at the positive pressure, NH3 easily attaches to the inner wall of the sampling pipe. In addition, the measurement of the concentration conducted in the measurement cell is a Fourier transform infrared spectroscopy (FTIR) or a Non-dispersive infrared analyzing method (NDIR). In order to conduct the measurement, it is required that the pressure in the measurement cell is made generally the same as the atmospheric pressure. With this arrangement, there is a problem that NH3 attaches to inside of the measurement cell. Furthermore, with this arrangement, since the exhaust gas introduced into the measurement cell passes a pump, there is also a problem that NH3 attaches to the pump.